In a case where a multivalued image is output as by a color ink-jet printer using inks of the four colors cyan (C), magenta (M), yellow (Y) and black (K), pseudo-halftone processing is executed using a method such as the error-diffusion method independently for each color. As a consequence, even though the visual characteristics are excellent when viewed for a single color, excellent visual characteristics are not necessarily obtained when two or more colors are superimposed.
For example, assume that an image is formed exclusively of dots based upon the C component and dots based upon the M component. If the original image is composed of 50% of the C component and 50% of the M component, then, ideally, all pixels will be filled with dots based upon C ink or dots based upon M ink, as shown for example in FIG. 17A. If, with the dots formed in this manner, the dot positions of the C ink and the dot positions of the M ink shift relative to each other for some reason, as shown in FIG. 17B, then the major part of the image will be dominated by pixels in which overlap occurs between the dots of the C ink and the dots of the M ink, i.e., pixels having a bluish appearance, and white pixels in which no dots are formed.
Accordingly, in a case where printing is performed using a printhead having a structure in which nozzles that eject C ink and nozzles that eject M ink are juxtaposed along the scanning direction of a carriage, as in ink-jet printers, the formed image will fluctuate periodically as in FIG. 17A or FIG. 17B, owing to a fluctuation in the scanning speed of the carriage, in accordance with position along the scanning direction, and the probability that white pixels will be present also will fluctuate. To the human eye, therefore, it appears that the density of the applicable area varies periodically. In other words, to the human eye, the image appears to be of poor quality.
By contrast, if an image is formed by laying out dots of the C ink and dots of the M ink entirely independently, then, in the case of an original image having a C component of 50% and an M component of 50%, as in the case mentioned above, ideally pixels that are not printed at all, pixels printed only by C ink, pixels printed only by M ink and pixels printed by a combination of both the C and M inks will be formed evenly at a probability of approximately 25% each, as illustrated for example in FIG. 18A.
Further, in the case where an image is formed by laying out dots of the C ink and dots of the M ink independently, there are also instances where a shifting of positions at which the dots are formed causes pixels that are supposed to be printed solely by C ink to be overlapped by neighboring pixels printed in M ink, as depicted for example in FIG. 18B. Conversely, however, there is also a possibility that pixels that are supposed to be printed by both the C and M inks will no longer be printed solely by the C ink or M ink, and hence the overall change in density is small in comparison with the case where dots of the C ink and dots of the M ink are laid out entirely exclusively.
Thus, though it can be said that laying out the dots of the C ink and dots of the M ink in exclusive fashion has the effect of reducing the graininess of the high-light portions of an image, a problem which arises is that in some cases the uniformity of the image tends to be lost in areas of intermediate to high density in a tradeoff with the precision of image formation. In regard to the high-light portions, however, the dots are already spaced apart from one another sufficiently and therefore a decline in image quality ascribable to shifting of dot positions is very small. This means that the effects of the exclusive-dot arrangement are more significant.
In order to deal with this problem, there has been proposed a method (see for example the specification of Japanese Patent Application Laid-Open No. 2002-171420) of correcting output values based upon input values and the total of the input values after two or more colors are subjected to pseudo-halftone processing independently, thereby applying a similar improvement.
With this prior-art method, however, the processing load is heavy because conditional branching is necessary at the time of binarization while pseudo-halftone processing is performed independently for each color. Further, as shown in FIG. 21, even in a case where an area to be controlled exclusively is partitioned using the value of each component, the difference between components and the sum of components, there is a possibility that the area will be partitioned linearly and that the separation obtained may not be suitable for each color component. Further, if the color space is split up too finely, conditional branching also becomes more complex and the processing load becomes increasingly heavy.